We don't need to include pg_sema.h in s_lock.h anymore.

Minor improvement to commit daa7527afc2274432094ebe7ceb03aa41f916607:
s_lock.h no longer has any need to mention PGSemaphoreData, so we can
rip out the #include that supplies that.  In a non-HAVE_SPINLOCKS
build, this doesn't really buy much since we still need the #include
in spin.h --- but everywhere else, this reduces #include footprint by
some trifle, and helps keep the different locking facilities separate.